Drill



M arch 6, 1945. R, w. ANDRl-:AssoN 2,370,706

DRILL Filed Nov. 29, 1945 INVENTQR.

2@ /6 l E E. FMOMMMMSM /rra my: ya.

Patented Mar. 6, 1 945 ofthe lands and in whichl the/relief is ofaA prefdetermined cross-sectional" configuration-,'- `the1 the landsthereonare-relievedv-onlybetween the opposite sideedges thereof; butasrfar as lam4 aware; in the past'such'reli'ef has taken the ,f'orm of' agroove located' centrally of the land' and provision ofl a .twist drillas= above described in which the configuration of the relief ispartiallyu t circular in section; and the provision-ofl a twist drill asabove described which the radius ofi-the relief` bears: a predeterminedrelation withl respect tozthe helix angle'andvdiarneter ofthe d'rillAThe above being among thefobjectsfof the'present"` invention, the samevconsists in certain novel featuresY of: construction tobe hereinafterdescribed with reference to ther accompanying drawing, and thensclaimed,havingV the above and other objects in view; l

In the accompanyingdrawing which illustrates suitable embodimentsofthepresent invention and in which like numerals refer to like partsthroughout :the-several.y dierentview Fig'. 1' isr a side.elevationalwiew ofra` twistdrill` embodying features of i the-f. presentinvention;

Fig. 2 is an enlarged cross-sectionalvviewftaken Figs. 3 and-learefviewssimilarz'to: Fig. 2', but illustrating the formf ofrelief of differenthelix angles.: i

Althoughpthe drill .of the presentzinvention is? particularlyadaptablefor use in rconnection with'- the drilling device. shown andclaimed in my application,forLet-ters Patent offll the United States ledSeptember 30, 1943, and serially numbered 504,351 and in which therelief in the lands of the drill is employed for conducting oil, cuttingcom pound or the like to the point of the drill during a cuttingoperation, I have found thatthis drill possesses certain advantagesregardless of its use. In other words, regardless of its use, it'will befound to be extremely resistant to vbreakage under torsion, it acts tocentralize itself in a hole being drilled, even tho-ugh improperlysharpened, to a much greater extent than conventional types of twistdrills, and the form of relief is such as to enable it to be readily,quickly, and cheaply formed by various types of apparatus.

It is not new to provide twist drillsin which employed. .for drills withtlieLbottom of the groove struck' on a" curve coincident with theaxisIof" thedrill. Ini sucl'r cases, the sidewalls oftherelief-grooveareapproximatelyf perpendicular to the bottoni' surface jthereof and the-I junction between the A"side Vand' bottom walls" of therelief groove-are relatively-- sharp, thereby engendering rupture atsuch point" when' the drill is 'subjected' to7 heavy' torsional loads.Ina'ccordancewith the-present' invention,

the relief groove-isfsdformed 'as to'v eliminate any such sharp` edgesYandiv itfha's" been found that y thereby the torsional strengtlrl of'thefrl'rill` is'4 greatlyV` increased In accordance withl the present'inventiom'thel relief tis curved and is 'preferalzilycircular incrosssectional congurationland'struck from atcenter"A located outsideoffthep'erlphery ofthe drill,.a radial' lineconnect'ing the vcenterofithe drill and the center of curvature ofthe relief passingjap`proximatelyv centrally through *the* lands" so that; the relief itselfterminates shortoftheomiosite'-v edges (of `the land; y This means'that' eachland? provides twoi angularl'y,` separated areas; of con',-vtact between the drill andthe work.,so 'that in a1" 36 conventionaltwo-ii'uted Vd'rillzthere are four areas?V of 'contact between vthedrill"and the walls ofthe bore itis drilling to" more accurately maintainthe", concentricity ofltlie "drill: anjd'the bore" regardless of errorsin sharpeningti'ie point" of the drill".v

The form of relief provided by the present invention provides a"construction in which the v edges of the lands are more stronglyvreinforced than where the conventionaltyp'eof landrelief'isf'Iemployed, and breakingorV chippingof l the 3 cutting*- edges along Atheadvanced edges'o'f the lands#l is: therebyv minimized? Thecross-sectional curvature ofthe relief in the lands in accordance withthe present inven-` tion may in the broader aspects of the inventionvary to a greater or lesser extent, but in accordance with a morelimited phase of the present invention, the curvature should bear adefinite relationship with respect tou the helix angle' of` the drillinorder to obtain optimum results. In

other words, yI have found that the greater the helix angle of the drillthe smaller the radius of curvature of the relief should be, and viceversa,

and that the radius of the relief should bear a definite relationshipwith respect to the diameter of the drill.k i

.Referring now to the accompanying drawing. i

about the'axis of the drill from the shank IIJ to the opposite end ofthe drill which is pointed and sharpened as at I6 in a conventionalmanner.

In accordance with the present invention, the lands I8 between theflutes i4 are centrally relieved as at and only between the oppositeedges of such lands. The relieved areas 2i) terminate a short distancefrom the opposite edges of the lands', which distance, of course, willvary in accordance with the diameter of the drill', but in all casesAwill leave a suicient amount of the lands to remain to provide a desiredamount of bearing surface between the drill and the bore being drilled.It will also be understood that if desired the unrelieved portion of theland on the trailing side thereof may be relieved a slight amount ascompared to the unrelieved portion thereof at the leading edge of theland as, for

, instance, by a matter of a few thousandths of an inch.

In accordance with the present invention, the relief 2D foreach land I8is of curved cross-sectional conformation so that it forms a concavegroove-like relief in the land. For manufacturing reasons, it ispreferably partially circular in sec-V tion with the center of curvaturelocated beyond the periphery of the drill and preferably substantiallyon a line passing through the axis of the drill .and midway between theopposite edges of the land as brought out in Fig. 2.

As previously stated, while in the broader aspects of the invention thecurvature of the bottom wall of the relief 20 may be varied to a greateror lesser extent in any particular drill, I have found, in accordancewith a more limited phase of the present invention, that the radius ofsuch relief, when circular, should bear an approximately definiteArelationship not only. to the diameterl of the drill, but to the helixangle of the drill in order to obtain optimum conditions. The curvatureof the relief in such cases may be approximately determined by thefollowing formula:

where R=the radius of the curvature of the relief in.VV inches, A=thehelix angle of the drill in degrees; and D=the diameter of the drill ininches.

The drill shown in Fig. 1 is intended to have a 35 degree helix angle ofthe utes I4 and lands l8. Applying the above formula to such drill, itwill be noted that the radius of the relief equals 45 minus 35 dividedby 10XD, or D. Accordingly, it will be observed that in accordance withthis formula the radius of the relief for a 35 degree helix angle drillshould be approximately equal to the diameter of thedrill, and this isthe condition illustrated in Fig. 2. The arc of such relief in suchcase, measured angularly about its center, and with a preferred amountof the land remaining for bearing against the walls of the `bore to bedrilled thereby, will be approximately 33 as shown in Fig. 2.

. On the other hand, applying the above formula to a drill having a 24degree helix angle as assumed in Fig. 3, then R under such conditionswill approximately equal 2D or will be approximately equal to twice thediameter of the drill. The arc of the relief in such case will beapproximately 16 as shown. On the other hand, Where a drill having a 15degree helix angle is relieved in accordance with the present invention,then as il lustrated in Fig. 4 and applying the above formula thereto itwill be notedthat the radius of relierr in such oase will beapproximately equal to three times the diameter of the drill. The 'arcof relief in such case will be approximately 11 as shown.

One fact will be appreciated by those skilled in the art and that isthat with the type of relief provided by the present invention therelief may be easily and quickly formed in thelands of drills withsubstantially no diilculty because of possible interference between therelieving tool and the Work.

Having thus described my invention, what I claim by Letters Patent is:

1. A'twist drill having a land relieved between the opposite side edgesthereof and being substantially unrelieved along said side edges, saidrelieved surface being arcuate in cross-sectional configuration andstruck about a center lying radially outwardly of the periphery of saiddrill,.

the approximate radius of said circle being calculated according to theformula 45-.4 R T X D where R is the radius of said circle in units ofmeasurement, A is the helix angle of said land in degrees, and D is thediameter of said drill in said units of measurement,

2. A twist drill having a concavity in a land thereof forming a reliefthe surface of which is partially circular in cross-sectional view andin which the radius of thek relief circle in inches is approximatelyequal to,l 45 minus the helix angle of the drill in degrees, divided byl0, and multiplied by the diameter of the drill in inches.

RUDOLF W. ANDREASSON.

